This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-254226, filed on Aug. 30, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a high-frequency power amplifier.
2. Related Background Art
The high-frequency power amplifier is used for supplying electric power to the antenna of a communication gear such as a portable terminal. The high-frequency power amplifier is required to have a high power gain and a high power efficiency and be as small as possible because it is used for the portable terminals and the like.
Recently, a high-frequency power amplifier incorporating a matching circuit has been designed. The matching circuit is used to match the impedance (normally 50xcexa9) of input/output between the high-frequency power amplifier and an antenna. Therefore, a small high-frequency power amplifier, which incorporates such an external circuit as the matching circuit as a module, is in demand.
To reduce the size of the high-frequency power amplifier, various kinds of methods are available, such as miniaturization of a semiconductor chip, intensification of the precision of mounting technology, refining the pattern of a module substrate. Particularly, because an output-matching circuit requires a large space, the miniaturization of the output-matching circuit provides a significant effect upon reduction of the size of the entire high-frequency power amplifier.
FIG. 10 shows the circuit diagram of a conventional high-frequency power amplifier 600. Its input-matching circuit is omitted from this diagram. A fundamental signal is outputted from a transistor Tr produced on a semiconductor chip 10. The fundamental signal is matched by a fundamental-signal matching circuit 20 having an inductor L1 and a capacitor C1 formed on a substrate. Ordinarily, the fundamental-signal matching circuit 20 matches the impedance of the fundamental signal to 50 xcexa9 and outputs the fundamental signal through the capacitor C4 for DC current block.
A power supplying circuit 30 comprises a capacitor C2, an inductor L2 and a direct current (DC) power supply 32 so as to supply power to the transistor Tr. The capacitor C2 is a decoupling capacitor provided for grounding the high-frequency signal. The inductor L2 is a so-called xcex/4 line which is set to xcex/4 with respect to the wavelength xcex of the fundamental signal. Consequently, the inductor L2 turns to a large impedance for the fundamental signal propagated from the transistor Tr to the fundamental-signal matching circuit 20. In other wards, the inductor L2 turns to open state to the fundamental signal at a node 50 in which the power supplying circuit 30 is connected to the fundamental-signal matching circuit 20. Therefore, the power supplying circuit 30 never affects the fundamental-signal matching circuit 20. The secondary harmonic signal of an output signal from the transistor Tr is short-circuited by the inductor L2.
The high-frequency power amplifier 600 further comprises a harmonic matching circuit 40 having an inductor L3 and a capacitor C3. The tertiary harmonic signal of the output signals from the transistor Tr is short-circuited by the harmonic matching circuit 40.
If the inductor L2 (xcex/4 line) is formed of a 50 xcexa9 line and the frequency of the fundamental signal is 900 MHz, the inductor L2 needs to be about 30 mm long. Due to a demand for reduction of the size of the high-frequency power amplifier in recent years, the high-frequency power amplifier 600 including the output-matching circuit 60 is formed in the form of a square module 4 mm to 6 mm in one side. Therefore, it is very difficult to accommodate the output-matching circuit 60 having the inductor L2 about 30 mm long in the module.
When the length of the inductor L2 is shorter than 30 mm (for example, shortened to 20 mm) in order to solve this problem, the power supply circuit 30 is not in the open state to the fundamental signal because the inductor L2 is shorter than the ideal xcex/4 line. Consequently, the electric loss of the output-matching circuit 60 is increased and the output impedance of the transistor Tr is changed.
When the inductor L2 is formed of a transmission line having a higher impedance than the 50 xcexa9 line, for example, when the width of the transmission line of the inductor L2 is reduced, the influence on the fundamental signal by the power supplying circuit 30 can be reduced. However, when the width of the transmission line is decreased, the resistance component possessed by the inductor L2 is increased, and thereby the power from the DC power supply 32 is lost. As a result, the power efficiency of the high-frequency power amplifier 600 drops. Further, because the voltage drop by the inductor L2 is increased, the voltage amplitude at the operating time of the transistor Tr is limited, and thereby the linear motion of the high-frequency power amplifier 600 is disturbed, and distortion component is increased in the output signal. The linear motion is important in digital modulation system. If the high-frequency power amplifier 600 is based on the digital modulation system, the increase in the distortion component leads to a large drop in the performance of the high-frequency power amplifier 600.
To solve such a problem, the high-frequency power amplifier 700 shown in FIG. 11 has been proposed. The output-matching circuit 60 possessed by the high-frequency power amplifier 700 comprises a parallel-resonant circuit 70 composed of a capacitor C5 and an inductor L4. If the resonant frequency of the parallel-resonant circuit 70 is set to the frequency (for example, assumed to be f0) of the fundamental signal, the impedance of the power supplying circuit 32 as viewed from the node 50 is increased. The relation between the capacity C5 of the capacitor C5, the inductance L4 of the inductor L4 and the angular frequency xcfx890 (=2xcfx84f0) of the fundamental signal is expressed in the expression 1.
L4=1/(xcfx8902*C5)xe2x80x83xe2x80x83(expression 1)
Because the value L4 can be set smaller by increasing the value C5, a small inductor can be used as the inductor L4. The inductor L2 does not have to be provided with a large inductance due to the effect of the resonant circuit. Consequently, the length of the inductor L2 can be shorter than the xcex/4 line under the condition in which the power supplying circuit 32 gives no influence upon the fundamental-signal matching circuit 20. Further, because the size of the inductor L4 is small, and because the length of the inductor L2 is shorter than the xcex/4 line, the high-frequency power amplifier 700 is smaller than the high-frequency power amplifier 600 and loss in DC current can be reduced.
However, in the high-frequency power amplifier 700, the power supplying circuit 30 is incapable of short-circuiting the secondary harmonic. The harmonic matching circuit 40 needs to be formed as a secondary harmonic matching circuit in order to remove the secondary harmonic from the output signal. As a result, the high-frequency power amplifier 700 is incapable of matching harmonic higher than secondary harmonic. Thus, in case of adjusting up to the tertiary harmonic like the high-frequency power amplifier 600 shown in FIG. 10, it is necessary to add other harmonic matching circuit to the high-frequency power amplifier 700. This increases the number of necessary components, which is contrary to the demand for reduction of the size. Adding another high-frequency matching circuit may cause a loss in the fundamental signal.
For the reason, there is demand for a high-frequency power amplifier having a high power efficiency and capable of matching secondary harmonic or higher order harmonic while the size thereof is smaller than the conventional example.
According to an embodiment of the present invention, there is provided a high-frequency power amplifier comprising: a transistor which is inputted with a high-frequency signal, amplifies the high-frequency signal and outputs the same; a fundamental-signal matching circuit one end of which is connected to an output of said transistor and which matches at least the impedance of fundamental signal in the amplified high-frequency signal and outputs the same from the other end; a power supply which supplies electric power to said transistor from a node located in an interval from the output of said transistor to said fundamental-signal matching circuit; a first inductor, one end of which is connected to said power supply; a second inductor connected in series between the other end of said first inductor and said node; and a first capacitor, one end of which is connected between said first inductor and said second inductor while the other end thereof is connected to a reference potential, said first capacitor forming a first series-resonant circuit with said second inductor and a parallel-resonant circuit with said first inductor.
According to another embodiment of the present invention, there is provided a high-frequency power amplifier comprising: a transistor which is inputted with a high-frequency signal, amplifies the high-frequency signal and outputs the same; a fundamental-signal matching circuit, one end of which is connected to an output of said transistor and which matches at least the impedance of fundamental signal in the amplified high-frequency signal and outputs the same from the other end; a power supply which supplies electric power to said transistor from a node located in an interval from the output of said transistor to said fundamental-signal matching circuit; a first inductor, one end of which is connected to said power supply; a second inductor connected in series between the other end of said first inductor and said node; a third inductor, one end of which is connected to said node; and a first capacitor, one end of which is connected to the other end of said third inductor while the other end thereof is connected to said reference potential, said first capacitor forming a first series-resonant circuit with said third inductor and a parallel-resonant circuit with said first inductor.
According to still another embodiment of the present invention, there is provided a high-frequency power amplifier comprising: a transistor which is inputted with a high-frequency signal, amplifies the high-frequency signal and outputs the same; a fundamental-signal matching circuit, one end of which is connected to an output of said transistor and which matches at least the impedance of fundamental signal in the amplified high-frequency signal and outputs the same from the other end; a power supply which supplies electric power to said transistor from a node located in an interval from the output of said transistor to said fundamental-signal matching circuit; a first inductor, one end of which is connected to said power supply; a second inductor connected is series between the other end of said first inductor and said node; a third inductor, one end of which is connected to the output of said transistor; and a first capacitor, one end of which is connected to the other end of said third inductor while the other end thereof is connected to said reference potential, said first capacitor forming a first series-resonant circuit with said third inductor and a parallel-resonant circuit with said first inductor.